1. Field of the Invention
The present invention relates to a magnetic recording medium having a high recording density which is applicable to various devices in the information industry.
2. Description of the Related Art
A coated type magnetic recording medium (which consists of a coating of uniformly dispersed, powdered ferromagnetic material such as .gamma.-Fe.sub.2 O.sub.3, Co-.gamma.Fe.sub.2 O.sub.3 or CrO.sub.2 in an organic binder on a non-magnetic substrate) has been used as a magnetic recording medium (e.g., a magnetic flexible disk, magnetic tape etc.). Now, in order to make a high density recording, a thin metal film type magnetic recording medium has been investigated as a replacement for the coated type magnetic recording medium. In the thin metal film type magnetic recording medium, a magnetic recording metal film is directly formed on a non-magnetic substrate by a plating method, a sputtering method, a vacuum deposition method, an ion plating method or the like.
When the thin metal film type magnetic recording medium is actually used, smooth runability is not obtained and results in an extremely unstable signal in recording and reproducing. The reason is that large frictional forces between the bare back face (i.e., the opposite face of the substrate to the face coated by the magnetic recording metal film) of the non-magnetic substrate and guides or posts. Thus, in actual use of the thin metal film type magnetic recording medium, a back coat layer having a low and stable coefficient of friction and having a strong wear resistance is necessary, and it is important to maintain these characteristics under bad environmental conditions in practice.
Examples of the back coat layer made of vinylchloride group or vinylacetate group are shown, for instance, in U.S. Pat. Nos. 4,567,083, 4,587,150, 4,618,535, 4,628,009, 4,639,389, 4,663,217, 4,673,622, 4,592,942, 4,618,535, 4,448,847, 4,789,583 and U.S. Pat. No. 4,786,557.
Examples of the same made of a polyester group or an acrylic group are shown, for instance, in U.S. Pat. Nos. 4,687,699, 4,637,963 and U.S. Pat. No. 4,443,514. Examples of the same made of a polyurethane group are shown, for instance, in U.S. Pat. Nos. 4,567,063, 4,612,244, 4,612,235 and U.S. Pat. No. 4,587,150.
These conventional back coat layer products showed improved runability, but undesirable exfoliation in running was observed. Thus, there were problems such as: wear resistance was insufficient, there was an undesirable transfer of component in the back coat layer onto the magnetic recording film and there was an undesirable transcription of shape of the back coat film surface onto the magnetic recording film surface.